The Influences Of Simulated Nitrogen Deposition And Understory Removal On Available N And P Dynamics In A Cunninghamia Lanceolata Plantation

Since the 1960s,global change phenomenon's such as atmospheric nitrogen deposition have emerged due to the rapid development of global economy and population growth,which have greatly influenced forest ecosystems.In this thesis,simulated canopy nitrogen deposition(25 kg N·hm^-2·a^-1),and understory removal treatments were conducted in the Cunninghamia lanceolata plantation based on the nest design.The dynamics of leaf nutrient resorption,litter decomposition,soil physical and chemical properties,soil nitrogen mineralization and soil microbial community structure?Phospholipid fatty acid method,PLFAs?were investigated with a viewpoint of"plant–soil–microorganism".The main goal of this study was to understand the effects of nitrogen deposition?N?,understory removal?UR?and their interactions on nutrient cycling and turnover in subtropical forest ecosystems,which would provide scientific theoretical basis for the sustainable management of Cunninghamia lanceolata plantations.The main findings are as follows:?1?The range of foliar nitrogen resorption efficiency was 19.04³5.00%,the foliar phosphorus resorption efficiency was 37.81⁶3.96%.Relative to control treatment?CK?,N and UR treatment significantly increased the foliar resorption efficiency of nitrogen and phosphorus.While N×UR treatment inhibited the transfer of foliar nitrogen and phosphorus.The foliar resorption efficiency of nitrogen and phosphorus was higher in rainy season than dry season.?2?When foliar litter was decomposed for one year.For the above four treatments?i.e.CK,N,UR and N×UR?,results showed that the average annual decomposition coefficients were 0.399,0.459,0.639 and 0.524,the turnover time were 7.51 year,6.53 year,4.69 year and 5.71 year for the treatments of CK,N,UR and N×UR,respectively.The decomposition processes showed a trend of"release?leaching?,enrichment and release"for carbon and nitrogen concentrations.There was a significant negative correlation between foliar C/N and dry matter residual rate,meant that the higher C/N value with lower decomposition rate.In short,nitrogen deposition and understory removal can promote the return rate of foliar nutrients.?3?N treatment significantly reduced the concentrations of soil organic carbon and total phosphorus,but had no effect on total nitrogen concentration.Soil C/N average value was relative stable with average value of 19.01.The value of soil C/P and N/P showed a seasonal variation with larger in rainy season than dry season.?4?The UR treatment significantly improved rates of soil net nitrogen mineralization,net nitrification and net ammonization,which increased the nitrogen leaching.Net nitrogen mineralization rate was highest in summer and followed by spring,autumn and winter.N and N×UR treatments increased the concentrations of NO₃^--N and NH₄⁺-N,but rates of net nitrogen mineralization and net ammonization were negative in these period,showing the sequestration of soil nitrogen.?5?UR treatment increased the quantities of soil microorganisms based on phospholipid fatty acids?PLFAs?.Although N and N×UR reduced total PLFAs but increased the proportion of bacteria and fungi to total PLFAs.The amounts of PLFAs in the rainy season was significantly lower than that in the dry season.Redundancy analysis showed that soil NO₃^--N positively related with PLFAs i14:0,15:0,i15:0,a15:0,16:1?7c,18:1?9c and18:2?6c.Soil available phosphorus positively related with PLFAs i14:0,i15:0,a15:0,16:1?7c and 18:3?6c.Nitrogen deposition and understory removal would change soil microbial community by changing soil available nitrogen and phosphorus.